Method of and apparatus for magnetically determining gas content



Aprll 12, 1949; HQRNFECK 2,467,211

METHOD OF AND APPARATUS FOR MAGNETICALLY DETERMINING GAS CONTENT Filed Feb. 24, 1944 2 Sheets-Sheer. l

DIAMAGNETIC PARAMAGNETIC i5.

) Snvwutor ANTHONY J. HORNFECK FIG. I v f April 12, 1949. v A. J. HORNFECK 2,467,211

METHOD OF AND APPARATUS FOR MAGNETICALLY DETERMINING GAS CONTENT Filed Feb. 24, 1944 v 2 Sheets-SheetZ VOLTAGE AMPLlFIER 8 l I M Q F ER a A u l rcfllrt 1 L Q WAVE FILTER ALN ICO 3nncntor ANTHONY J. HORNFECK (lttorncg earths atmosphere.

Patented Apr. 12, 1949 METHOD OF AND APPARATUS FOR MAG- NETICALLY DETERMINING GAS CON- TENT Anthony J. Hornfeck, Cleveland Heights, Ohio,

minor to Bailey Meter Company, I corporation of Delaware Application February 24, 1944, Serial No. 523,755

17 Claims. 1

This invention relates to the determination and measurement of a constituent gas in a gaseous mixture. For example, the detection and measurement of the percentage content, of free oxygen in the products of combustion of a fuel burning furnace, an internal combustion engine, or the like.

My invention is based primarily on the theory that different gases, have different magnetic permeabilities and that the magnetic permeability of oxygen may be used to determine its presence and proportion in a mixture of gases or fluids.

It is a well known fact that oxygen is magnetic to a far greater degree than almost any other gas. This is particularly true of oxygen in gas mixtures such as combustion flue gases or the In the language of the physicist oxygen is quite paramagnetic or has a positive magnetic susceptibility. This means that the magnetic permeability is greater than unity.

where permeability and K-=susceptibility The following table compares oxygen with several other gases:

- This table shows that the susceptibility ofoxygen suspended. Conversely, the amount of oxygen in an unknown gas can be determined from the force or torque acting on a suspension carrying a sphere filled with a reference gas. The sphere in this case is suspendedin space occupied by the unknown gas. Such a device is similar to a sensitive galvanometer but differs in that its deflection measures permeability rather than small currents or potentials. Since the forces involved in such a device are exceedingly small, it follows that the instrument is necessarily rather fragile and subject to shock, vibration, etc.

Attempts to measure oxygen magnetically by various other electrical systems have not been successful. Schemes suggested include bridge or potentiometric circuits which compare the inductances of coils and oscillators whose frequency depends on the inductance of a coil or the mutual inductance between two coils. In such comparison schemes:

' Output- F1 (G1) rm-F1 (Ga) ftp-2 where F1 (G) means a function of the geometry of the coils 1=permeability of measured gas m=permeabi1ity of referencegas The slightest change in the geometric configuration of one coil'relative to the other will therefore produce tremendous errors of measurement.

The obvious difllculties involved in these systems suggested that a system in which the output is a more direct function of the difference in permeability between a reference and a measured gas would be more practical; for example, where Output=F (G) (;up2) (3) the magnetic path when a salient pole iron rotor is turned in the magnetic field. In my invention of a gas analyzing device all iron parts are rigidly fixed while a disc having slots or holes and made of a non-magnetic material such as quartz, glass or a plastic is spun at a relatively high velocity in the air (gas mixture) gap. Since the magnetic field must be held constant, it can be produced by a coilenergized from a battery or preferably by a permanent magnet. The high energy magnetic materials such as Alnico permit a high flux density to be maintained in the air gap with a moderate size of magnet.

The magnetic flux passing between the poles and linking the coils alternately passes through a gap filled entirely with sample gas mixture and then partially filled with the disc material. As a result, the magnetic permeance oi the gap varies periodically an amount depending upon the difi'erence in permeability between the disc and'the I A 7 gas. It the magnetomotive force across the gap is fixed, the magnetic flux will vary periodically resulting in an induced voltage in the coil.

In the drawings:

Fig. 1 is a partially cut-away front elevation of a gas analyzer embodying my invention.

Fig. 2 is a. sectional view along the line 2-2, in the direction oHhe arrows, of Fig. 1.

Fig. 3 is an elevatlonrirrdiagrammatlc form, of another embodimen taoi myinventlon.

Fla, 4 is a wiring diagram 01 a measuring arrangement including the apparatus of Figs. 1 and 2. H\

5 isa wiringsi lsmmincludlhg a compensating circuit.

Fig. 6 diagrammatically ates an analyzer including a compensating genera Fig. 7 is a sectional elevation oi) a lhmlnated disc. K/MWM Referring now to Figs. 1 and 2, I show therein a gas analyzer embodying my invention. A housing I includes a pair of similar coil mounting plates 2 spaced, by a closed magnetized ring 3 01 Alnico metal. Each'platel has mounted thereon eight coil windings 4 equall spaced about a center. The mounting plates are-of soft iron and each coil is supported by a soft irheore s. The coils of the two sets are aligned and the aligned pole pieces are spaced approximately one-eighth inch. In this air gap is a rotatable disc 6 having eight equally spaced elongated fingers 1. Provision is made for rotating the disc at a relatively high velocity by a motor 8 (Fig. 4) which may be directly connected or belt connected. The housing I is preferably gas tight and provided with a gas inlet 9 and outlet (0. The necessary filters. cleaners and pump are provided to supply a continuous fiow of gas mixture (such as combustion fiue gases) through the housing I.

If the permeability of the disc is unity, it can be shown that the voltage generated in the coils is a direct function of the oxygen content of the Let e=generated voltage per coll E=total voltage generated by coils in series F=magnetomotive force across the gap ao=permeability of plastic disc=1 (assumed) g=permeability of gas being analyzed N=number of coils P=number of poles=no. slots=N/2 1l=number of turns per coil f=frequency of generated voltage t=time s=speed of disc in R. P. M.

=fiux linking coils By fundamentals e n g? 10 and E== N" 3- 10- I also a-% where Pa=permanence oi the gap with .slot or hole disc in line with poles A=eflective cross section of pole taco G=eiiective length oi gap P0=permeance of the gap with finger or sciid porti of disc in line with poles W=t ckness of disc The total variation in permeance from slot in line with poles to finger in line with poles Ls AP=Pg-Po --ubstituting ior Pg and Po in (8) yields G W (my-n +M Since ,u-fl-pO is very small Equation 9 can he Written in simplified form The change in fiux linking the coils coring to this change in permeance is Slurp The time in seconds required for the change is The average voltage generated in a coil is.

Kg=susceptibility of the gas.

It may be noted here that the magnetic susceptibility of the gas is proportional to the mass or number of molecules of 02 present lira cubic cm. and not the percent by volume of the 02 in the gas.

% 0, by volume= guns. of 0, per cu. cm. in ghslnixtura density of O, at test press. and temp;

The frequency of the generated voltage is made as high as possible to permit efficient amplification and easy filtering of extraneous voltages produced by pickup and vibration. The equation for frequency is:

As an example I will now calculate the output of the generator shown in Figs. 1 and 2.

N=16 coils n=6000 turns P=8 poles F=1200 gilberts (approximately) B=10,000 R. P. M.

For 100% 02 at standard press. and temp.

Kg=1.95X10 =6n, if ao=1 Substituting these values in Equation yields 1.27 X 1.95X 10-X8 X 10,000=.0155 volts and by Equation 16 Since the energy output of the generator is quite small, it cannot be measured accurately with commercial instruments. However, the output is far greater than the minimum voltage which can be successfully amplified by commercial vacuum tube amplifiers of the audio frequency type. Certain well known arrangements such as regulated voltage supply and inverse feed back can be used to make the amplifier output a stable function of the input. The schematic diagram of a complete measuring system is shown in Fig. 4.

The purpose of the wave filter is to eliminate the frequencies produced by vibration of generator poles, pickup, etc. and to pass only the band of frequencies generated by the disc 6. The purpose of the inductor L in series with the output meter is to make the output insensitive to fluctuations in generator speed. That this is so can easily be shown as follows:

Assume that the reactance of the inductor is high compared to the meter resistance. Then the current flow through the meter will be a function only of the voltage output and the inductor reactance.

= 1330 cycles per second where K1=constant =speed'as before 6,u=,ug-p0 as before E=voltage output K1, K2 and K0 are constants In the present discussion it has been so far assumed that the material of the disc 6 has exactly zero magnetic susceptibility, or unity permeability, so that the voltage generated by the rotating disc would be zero in a vacuum and substantially zero in a gas containing no oxygen. Actually the material will be either paramagnetic or more probably diamagnetic which will result in a generation of voltage with no oxygen present in the gas passing through the gap between poles. There are many insulating materials either plastic or ceramic which have a mass susceptibility far less than oxygen, but being solid they may have a corresponding volume susceptibility considerably greater than oxygen. The following table illustrates this:

This shows that if a celluloid disc were used, the voltage generated with zero oxygen in the sample gas would be of the voltage generated with oxygen at standard pressure and temperature. There are a number of ways to compensate for this effect and obtain zero reading on the output meter for zero oxygen.

Fig. 5 illustrates what I believe to be the simplest method. It consists in introducing a compensating voltage of opposite polarity into the output of the amplifier in order to exactly cancel the amplifier output at zero oxygen.

Fig. 6 illustrates the application of a compensating generator operating in a vacuum or inert gas such as nitrogen. The compensating generator I I being identical and synchronized with the test generator I will have an output exactly equal to generator I at zero oxygen. I

My preferred method of compensation is to compensate the rotor itself to reduce its effective magnetic susceptibility to zero. Refer to Fig. 7 A laminated disc rotor I2 may be used made up of a section of paramagnetic material banded with a section of diamagnetic material. The thickness of the sections are proportioned to produce zero total susceptibility.

1 1" 2 2 1+tz (18) where Ko=equivalent susceptibility of laminated Inspection of Equation 18 shows that Z Note: ILo=1+41rKo=1 when Ko=0 In the arrangement of Fig. 7 a thin coating or painting of a substance on the disc may be sufficient if the susceptibility of the substance is considerably greater than that of the disc. An

K =0 when gexamplev would be 'alacquered coating of ferric oxide (K =+108 10- on a Celluloid disc In this way a single disc could be made up of a mixture of paramagnetic and diamagnetic materials to produce a resultant susceptibility of zero.

If the disc has a very high electrical resistance, electric charges may be produced on the surface of the fingers which will induce voltages in the coils by electrostatic induction. Such voltages will have the same frequency as the voltage generated by the magnetic fiux pulsation and hence cannot be eliminated by electrical filters.

A solution for this problem is to paint the surface of the rotor with a very thin coating of semi-conducting paint such as colloidal graphite and to wipe off the charge with a brush made of fine copper wire. The disc itself might also be made of a semi-conducting material.

I have illustrated and described a. preferred embodiment of my invention. Another form or arrangement is shown in Fig. 3. Therein the cores I3 and I4 supporting the coils may themselves be of Alnico metal.

In addition to its usefulness in the analyzing of gaseous mixtures I contemplate that my invention is valuable in analyzing plastics and other substances for the presence of iron or other ferromagnetic materials. Since the magnetic susceptibility of iron is several million times that of the disc material, extremely small traces will generate substantial voltages. The instrument would first be calibrated by rotating a disc 6 of known composition in the atmosphere. Dimensionally similar sample discs of the material to be tested would then be rotated in the atmosphere at the same speed or speeds.

While I have illustrated and described certain preferred embodiments of my invention it will be understood that they are by way of example only and that I am not to be limited thereto except as defined in the following claims.

What I claim as new, and desire to secure by Letters Patent of the United States, is:

-1. The method of determining the percentage of an element in a mixture of elements which includes, passing the elements free of the constituent element through a magnetic field, varying the permeability of said field at a relatively high frequency, generating an electrical potential by the magnetic field while its permeability I is varied, introducing an electrical potential which balances out the potential generated, passing the elements with the constituent element through said field during continued variation of its permeability, and then measuring the electrical potential generated.

2. The method of determining the percentage of a constituent gas in a gaseous mixture which includes, passing a gaseous mixture which is free to vary the permeability of the latter at a relatively high frequency, generating an electrical potential by the magnetic field while its permeability is varied, introducing an electrical-potential which balances out the potential generated, passing the mixture intermittently through said field at the same frequency, and then measuring the potential generated as an indication of the percentage of the substance to be determined.

4. The method of determining the percentage of a constituent gas in a mixture with a known gas which includes, introducing the known gas -in amounts varying in a predetermined manner into a magnetic field, generating an electrical potential by the magnetic field while its permeability is varied, introducing an electrical potential which balances out the potential generated, introducing the gaseous mixture in amounts varying in the same manner into the magnetic field, and then measuring the electrical potential generated as an indication of the constituent gas present,

5. The method of determining the percentage of free oxygen in a mixture with known gases which includes, introducing into a magnetic field the known gases in fixed'percentage amounts but varying in volume in a predetermined manner so as to vary the permeability of the field, generating a voltage by the field which changes in intensity as a result of its varying permeability, introducing an electrical potential which balances out the potential generated, introducing said gaseous mixture in amounts varying in the same manner into the magnetic field, and then measuring the electrical potential as an indication of the free oxygen present.

6. The method of determining the presence of a gas of high magnetic susceptibility with gases of low magnetic susceptibility which includes, passing the gases of low magnetic susceptibility, with possible traces of the gas of high magnetic susceptibility, through a magnetic field, varying the permeability of said field at a relatively high frequency, generating an electrical potential by the magnetic field while its permeability is varied, and measuring the potential generated as an indication of the gases present.

7. In a gas analyzer in combination, a housing, means for circulating a gas to be analyzed through said housing, a plurality of opposed pole pieces mounted in the housing in the path of the circulating gas and spaced from each other to provide a gap therebetween, a rotatable fingered disc of non-magnetic material located in the gap between said opposed pole pieces, means for establishing a magnetic field between said opposed pole pieces, means for rotating said disc to pass the fingers thereof through said field and efiect a variable field intensity, means in which a voltage is induced by the varying field intensity, and means for measuring said induced voltage.

8. In a gas analyzer in combination, a housing, means for circulating a gas to be analyzed through said housing, a plurality of spaced opposed pole pieces each having a wound coil and mounted in the housing in the path of said gas, means establishing a magnetic field between said opposed pole pieces through the moving gas, said gas having a magnetic permeability varying with its constituent content, means passing intermittentlythrough said field for varying its intensity at a high-frequency to generate an electrical potential in said coils varying with said permeability as a determination of the composition of the gas, and means for measuring the potential generated.

9. In a gas analyzer in combination, a housing, means for circulating a gas to be analyzed through said housing, a plurality of opposed pole pieces each having a wound coil and mounted in the housing with a substantially uniform air gap between opposed pole pieces, the moving gas filling said gap and having a permeability varying with the percentage composition of the gas, a movable non-magnetic member which is operable when moved to pass periodically through said gaps between opposed pole pieces, thus presenting to said field alternately a path through the gas and a path through said member, means for moving said member at a relatively high speed whereby an electrical potential is generated in said coils varying with magnetic permeability of the gas and thus with the composition of the gas, and means for measuring the electrical potential generated.

10. In a gas analyzer in combination, means for producing a magnetic field, means for passing a sample flow of gas to be analyzed continuously through said field, the magnetic permeability of said gas stream depending upon the composition of the gas, means passed intermittently through said field for varying the field intensity at relatively high frequency, means in which an electrical potential is generated by the varying field in magnitude depending upon the said permeability, and electrical measuring means sensitive to said potential.

11. In a device for determining the percentage of tree oxygen in a continuously flowing stream of gas mixture, in combination, means for directing magnetic flux from a constant source across said stream, the resulting magnetic field strength varying with magnetic permeability of the stream in dependence upon the percentage of free oxygen in the mixture, means for varying the reluctance of said field at a high frequency to efiect a variation in its intensity, means in which an electrical potential is generated by the varying field, and means for measuring the potential generated.

12. In a device for measuring a gas which is present in a mixture 01' gases, in combination,

means for producing a magnetic field, means for introducing consecutively gases free or the gas to be measured and the mixture or gases into said field, means for varying the intensity of said field at a relatively high frequency while the gases are supplied thereto, means in which voltages are induced by the field of varying intensity, means for balancing out the voltage induced when the gases tree 01' the gas to be measured are introduced into said field, and means for measuring the voltage induced in said means in excess of the voltage balanced out.

18. The combination of claim 9 including electric measuring means tor the potential generated, an electric circuit including said measuring means, and a compensating electric potential generator connected to said circuit, said compensating generator having a field of fixed magnetic permeability.

14. The combination of claim 9 including electric measuring means for the potential generated, an electric circuit including said measuring means, and compensating means for said circuit whereby the measuring means is unafiected by susceptibility of the moving members.

15. In combination, an electrical measuring circuit, a pair of electric generators connected in said circuit in opposition to each other, each of said generators having a magnetic field path, means for directing magnetic fiux from constant sources across the field paths of said generators, means for introducing into the field path of one of said generators a mixture of gases including a gas to be measured, means for introducing into the field path or the other of said generators a gaseous mixture free of the gas to be measured,

means for varying the reluctance of said generator fields at a comparatively high frequency to vary their intensities, means in which voltages are generated as a result of said varying field intensities, and means for measuring the differences between the voltages generated in said last mentioned means.

16. In a device for detecting the presence of ferromagnetic metals in minute quantities in a substance having otherwise substantially zero susceptibility, in combination, means including a fixed field source for producing a magnetic field, means for passing the substance intermittently through said field and thereby varying its reluctance and effecting a variation of its intensity, means in which a voltage is generated by the field of varying intensity, and means for measuring the voltage generated.

17. As an article of manufacture, a fingered disc made up of layers of paramagnetic and dia magnetic materials of suificient thickness to produce a total magnetic susceptibility of zero.

ANTHONY J. HORNFECK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 421,479 Brace Feb. 18, 1890 1,640,524 Augustine Aug. 30, 1927 2,110,144 Durkee et a1. Mar. 8, 1938 2,237,254 Broekherysen Apr. 1, 1941 2,315,045 Breitenstein Mar. 30, 1943 2,405,137 Gale et a1. Aug. 6, 1946 2,416,344 Pauling Feb. 25, 1947 FOREIGN PATENTS Number Country Data 284.80! Great Britain Jan. 29, 1020 

